The invention relates to a process and device for checking substrate wafers, platelets or similar components which contain defects, and concerns in particular, a process and device for checking polished wafers of single crystal material with the aid of a microscope.
Platelets or wafers of single crystal materials, such as gallium-gadolinium garnet (GGG), are employed as the substrate in the epitaxial deposition of magnetic garnets. The required surface finish, however, can be achieved only by means of a very extensive polishing process in which the reproducibility of results does not depend finally on the concentration of dust particles, impurities in the polishing materials, or similar factors. The main difficulty usually lies in the presence of submicroscopic scratches which can not be seen, even at a magnification of several hundred times, without etching the surface.
The final examination of the wafers is a very important step in the whole manufacturing process. For this reason, both sides of the wafer are etched before inspection, since the defects become only then visible. The etched wafers are then usually examined for etch pits or scratches at magnifications of 100-200 times using incident light, and NORMARSKI microscopy, an optical inspection technique utilizing interference contrast. This manual inspection is very time-consuming since, at a magnification of two hundred times for example, only a very small area can be observed. Also, as is well known, microscopic observation of a moving field over a long period of time makes heavy demands on the viewer's concentration and is therefore extremely tiring.
Previous inventors have attempted to cope with this problem. For example Sawatari, U.S. Pat. No. 4,017,188, teaches an arrangement for measuring the profile of surfaces having a characteristic one-directional lay with sufficient resolution to determine the surface roughness. The surface is optically scanned with the aim of having a profile of the surface, i.e. a linear graphical profile of the surface is obtained by recording the signals as a function of the scanned distance.
Ash, U.S. Pat. No. 3,836,787, relates to apparatus for examining the surface of an object using electromagnetic radiation. The resolution is not limited by the wavelength of the radiation. The object has points to be determined which are smaller than the resolution of the apparatus. The surface of the object includes a plate having a small aperture in the centre of the field of view. The object must be vibrated relative to the plate having the aperture, so that radiation reflected from the object is modulated with the frequency of the vibration, namely the character of the reflected radiation differs from the character of the incident radiation.
Nisenson, U.S. Pat. No. 3,782,827, teaches an optical device which is useful for characterizing the surface topography of an opaque sample through the use of the sample's power spectrum, using light which is at least partially coherent. If one has none-opaque samples it is not possible to determine the morphology of the surface, but only variations of the refraction index.
Kojima et al., U.S. Pat. No. 4,030,837, teaches a method for measuring the reflectance of coals, including the provision of a movable sample stage below a microscope, and utilizing a combination of a microscope and photomultiplier. In converting the reflected light to an electrical output, Kojima integrates the electrical output.
The prior art thus discloses either an arrangement for measuring the profile of a surface by means of a light detector adapted only to measure the light distribution of light intensity without mapping point defects, as in Sawatari, requires an apparatus where the radiation from the reflected object must be different than the radiation incident on the object, as in Ash, characterizes properties of the sample through the uses of the sample's coherent power spectrum, as in Nisenson, or automatically measures the distribution of reflectance of coals, by integrating reflected light from a sample, and then indicates the distribution of reflectance, as in Kojima et al. None of the above references teach, however, a process for checking substrate wafers or the like for defects by optically sensing light differences between portions of an image free from defects, and portions of the image showing defects, and displaying the processed information in two dimensions on a display device.
Although attempts have been made in registering merely number of defects, there is no value, however, in registering only the number of defects, for example by counting the number of defects automatically, or otherwise, since their distribution is also very important. Consequently, a high local concentration of defects can be classified as a single defect when considering the extent to which the defects cause degeneration of the magnetic epitaxial layer.